ME 6930: Micro Fabrication Class notes of 02/27/2014Substrative ProcessesIn general ICs are manufactured layer by layer addition till the required number of transistors are fabricated.This is done by diffusion and ion implant methods. In diffusion, oxide layers are grown substrate with application of heat. Then dopants ( p-type/n-type) are added to substrate.Types of substrative process: Dry etching and wet etchingDry etching: The picture shown contains four examples of dry etching technique.A ) Metallized lines in passivation method:We have 11 layers in this picture. First insulator is applied on substrate. Then patterned with photolithography.B ) MEMS Comb driveIt is capacitance accelerometer. The distance between arms/finger changes capacitance and causes change in output voltage.It has suspended structure. It is fabricated layer by layer, followed by etching to remove material between layers to suspend.If etching is not proper, the layers will sandwich.They have moment of inertia very low. So the layers should be made stiffer.Anisotropic EtchingThe etching rate is different in different direction where as in isotropic etching, etch rate will be same in all direction. Etching will be drastic in particular direction.With anisotropic etching, vertical walls can be fabricated.Curved profiles and undercuts can be formed.First photoresist mask is applied on substrate. Then etched with etchant and again substrate applied over etched layer, patterned with photolithography. This is continued till required layer is obtained.The chemical used for etching attacks substrate strongly than photoresist. If chemical is left for long enough, all the substrate will be removed.The photoresist mask should be easy to remove after etching. If its strong, it will require plasma treatment for removal. Choose resist, such that it doesnt affect etching of substrate and easy to remove.Picture:The structure is formed by isotropic etching. The required profile depends on etch rate of chemical/gas used.In wet etching, pattern should be narrow to avoid undercut.Agitation of etchant is required to change the profile and control depth.The top layer is silicon oxide and bottom layer are silicon. This is because, most etchant attack only silicon oxide. Certain etchants attack certain planes of silicon faster. This can be used to form pyramid, triangle structures.Dry etchingIons/radicals are bombarded on substrate to remove material.The following are three possible cases of dry etching:1. Physical sputtering/ion etchingIon will sputter the surface. The material removal depends on the velocity of ions. Ions stream is used. The mask should be able to shield the ions in the unexposed region.

2. Chemical plasma etchingNeutral radicals are bombarded. They will react with substrate, form volatile products and live the substrate.Etching rate depends on affinity of substrate for radicals.3. Reactive ion etching (Physical/chemical etching)

Its combination of physical and chemical etching. Ions weaken the bonding in the substrate. Neutral radicals form volatile products aided by ion beam.

To prevent walls from etching, inhibitors will be used on the walls. Inhibitors prevents reaction between radicals and substrate.